Radar level gauging (RLG) to measure the level of a filling material, such as a liquid or a solid like a granulate is an increasingly important method for level gauging in tanks, containers, etc. The process of radar level gauging generally can be divided into three steps:                Searching: finding the correct surface echo among possible disturbing echoes.        Tracking: following the found surface echo during various changing conditions.        Accuracy: requirements for accuracy during typical tank conditions which typically are much more diversified than the usually cited “instrument accuracy”.        
For radar level gauging the conditions are very different for different applications. For instance the mm-accuracy in CTS-applications (custody transfer system) requires a very strict control of installation conditions. Such accuracy is nowadays practically feasible in marine and refinery installations but generally not easy to obtain in typical process applications due to the abundance of disturbing echoes. However, more or less all present RLGs on the market are optimized under the assumption of an easy echo situation where the surface echo generally can be distinguished by its strength. That is a natural development from the oldest RLG applications like the marine use where installation conditions are somewhat controllable and the CTS application in big refinery tanks where the distance to disturbing objects typically is as big as desired. In typical process tanks the distance to the walls is much smaller and a turbulent surface is a normal situation. Thus, the task of finding and tracking the right surface is much more difficult in a process tank than in for instance a refinery tank. From that background, it can be questioned whether an echo finding logic based on a echo logic for relatively easy conditions, improved by a number of more or less concurrent improvements really can be sufficient as compared to a logic directly developed for the actual situation. p Present RLGs typically use the information in FFT spectra to distinguish which echo corresponds to the surface to be measured. The FFT spectra reflects the reflected energy at different distances. Consequently, the FFT spectra is normally a rather inaccurate means for distinguishing between e.g. a stationary object, a turbulent filling material surface and a moving agitator structure arranged in the tank. Further, it is normally difficult to distinguish the filling material surface when the tank is essentially empty and is starting to be filled. At this time, the filling material surface is also rather turbulent. Still further, strong echoes from stationary objects and structures in the tank would intervene in the measurement, and the tracking functionality could easily be fooled to lock on an erroneous echo, or lose the filling material surface echo when close to a disturbing echo.
In existing RLG systems, different hardware related means have been proposed to select and maintain the selection of the right surface echo. For example, a narrow antenna beam reducing the amplitude of echoes from disturbing echoes such as constriction steel beams etc. have been used. Unfortunately, a more narrow antenna beam requires a bigger antenna diameter which may be incompatible with existing mounting holes. Consequently, the number of disturbing echoes occurring in the raw signal generally is bigger than desired in typical process tanks. Further, a range gating function is often normally implemented in the software. Regardless if the system is of FMWC or pulsed type, a “tank spectrum” can be used as a tool for selection of the most likely echo. The tank spectrum is a detected tank signal where the amplitude is only used in combination with suitable logical decisions. Still further, a function for measuring the amplitude and a related threshold is conventionally needed to discriminate noise or irrelevant echoes from the true surface echo. This is also mainly a software function but needs certain calibration of amplifications etc. in order to tell the system what a “normal” echo should be. A more or less sophisticated echo logic could also be used in processing the echoes which have passed the test with sufficient amplitude and sufficient similarity to already verified echoes. However, there is still a need for improved radar level gauging, especially for tanks having interfering structures generating reflecting signals.
In order to improve the situation various solutions have been proposed. Receiving the echo in two polarizations (with a slightly more complicated antenna/microwave module) and comparing the received signal is one method to increase the ability to distinguish a surface echo from less symmetric disturbing echoes. In this respect, see e.g. U.S. Pat. No. 6,759,976. The time variation of the echo amplitude is another way which may be combined with the polarization and which is useful for a turbulent surface. A third method is to use a still more complicated antenna for creating a few different antenna lobes (or a slightly non-vertical lobe rotating around the plumb-line) all in the ideal case giving the same surface echo but greatly different echoes from disturbing structures which typically are non-symmetric located. In this respect, see e.g. U.S. Pat. No. 6,759,977. These methods may be efficient but unfortunately they require extra hardware, and is therefore normally more expensive and more cumbersome to produce and install. Further, such system will normally require more time and development resources.
In order to improve the situation various solutions have been proposed. Receiving the echo in two polarizations (with a slightly more complicated antenna/microwave module) and comparing the received signal is one method to increase the ability to distinguish a surface echo from less symmetric disturbing echoes. In this respect, see e.g. U.S. Pat. No. 6,759,976. The time variation of the echo amplitude is another way which may be combined with the polarization and which is useful for a turbulent surface. A third method is to use a still more complicated antenna for creating a few different antenna lobes (or a slightly non-vertical lobe rotating around the plumb-line) all in the ideal case giving the same surface echo but greatly different echoes from disturbing structures which typically are non-symmetric located. In this respect, see e.g. U.S. Pat. No. 6,759,977. These methods may be efficient but unfortunately they require extra hard-ware, and is therefore normally more expensive and more cumbersome to produce and install. Further, such system will normally require more time and development resources.
Further, it is known, e.g. from EP 1 128 169 to use difference signals in order to improve the resolution or accuracy of RLG-systems. However, these known methods are proposed for different reasons than as a remedy to the above-discussed problems.
Thus, there is still a need for an improved RLG system that could alleviate the above-discussed problems.